Relative strength of acids and bases:
According to Bronsted, the strength of an acid (Relative strength of acids and bases) is measured from its tendency to denote a proton and that of a base from its tendency to accept a proton.
Strengths are generally expressed in term of dissociation constants (ka) and (pka) values of an acid.
The strengths of acid and bases is measured quantitatively by a degree to which reactants are converted to products in a chemical reaction.
In other words, the strength of an acid is measured by the position equilibrium . When a monoprotic acids, HA , dissolve in water. It donated its protons to water( a bronsted base) to form hydronium ion (H30+) and a conjugate base (A-) and the following equilibrium established.
HA + H2O <………> H3O+ A-
According to Law of Mass Action:-
k= [A-][H3O+] / [HA][H2O]
Where K is equilibrium constant and the quantities written the square brackets denotes molar concentration and to more exact thermodynamically, activities of reactants and products.
In dilute solution , the concentration of water molecules [H2O] , remain essentially constant.
Thus we have
K[H2O] = [H3O+][A-]/ [HA]
Ka = [H3O+][A-]/ [HA]
since K[H2O]= Ka
Relative strength of acids and bases Dissociation constant:
- Where K is the acid dissociation constant and represents the extent to which an acid id dissociated.
Therefore ,the values of Ka for a particular acid is a measure of its acid strength. The large the values of (Ka) stronger the acid.
- The values of Ka for this type of reaction also gives us information about the relative strengths of the two acids in the equilibrium , HA and H3O+ the conjugate acid of water.
- The values of acid dissociation constant (Ka) also indicates the relative base strength of conjugate base (A-).
As the values of the Ka of HA increases , tje base strength of (A-) decreases.
- The (1/Ka) represents the strengths of the base (A-). Generally, “the heavier the acid is, the weaker the base is to conjugate.
HCl is stronger acid as it has a greater tendency to donate a proton. But its conjugate base (Cl-) ion is a weak base as it has little tendency to accept a proton.” The stronger a base is the weaker its conjugate acid’‘.
* Numerical values of Ka are often very small numbers and it is convenient to express the acid strength as;
pKa = -log10Ka
The symbol (-log10) is by convention ‘p’.
* The larger the values of pKa , the weaker be the acid and vice versa.
Then HCl (Ka=107 and pKa = -7.0) is a stronger acid then
HNO3(Ka =103 and pKa=-3).
Aqueous solution of a strong acid in Relative strength of acids and bases:
*In any aqueous solution of a strong acid. All the original acid (HA) is dissociated and the values of Ka is large. On the other hand , a weak acid in aqueous solution is dissolved to a very small extent and the values of Ka is also small.
Thus in general we can say that the values of Ka is large for a strong acid while it is small for a weak acid.
Apparent strength of a protonic acid:
*The apparent strength of a protonic acid is dependent upon the solvent in which the acid is dissolved. If for a given acid we wish to increases the acid strength, the we should choose a solvent which has greater affinity for proton then has water.
* If we ad ammonia to a solution of HCl in water , the equilibrium is
H3O+ + NH3<…….> H2O + NH4+.
and clearly here NH3 has strong affinity for proton then water.
Since NH3 is a stronger base. Hence, if we dissolve an acid which is weak in water , in liquid ammonia the strength of an acid is increased (i.e) pKa decrease.
The formic acid is a weak acid is a weak acid in water but a strong acid in water but a strong acid in liquid ammonia.
*When we use any material for a protonic acid as a solvent, the acidic and basic species formed by dissociating the solvent molecules. Determine the limits of acidity or basicity in that solvent. Thus in water, we cannot have any substance or species more basic than OH- or more acidic then H3O+ in liquid ammonia.
* The all the strong acid like ,HClO4 , HCl , HNO3 and H2SO4 in water in water have very close pKa, values.
They appear to have nearly equal strengths because their strengths are “levelled” to that of the hydronium ion, H3O+. This phenomenon is called “levelling affect”.
- All the acid which are completely dissociated in aqueous solution are represented by H3O+ ion.
It is not possible to determine the order of increasing strengths of these strong acids in water because they are completely ionized. When these acids are dissolved in anhydrous acetic acid however significant difference in their acid strengths are observed.
Relative strength of acids and bases of protonic acids:
*The relative strengths of certain protonic acids are in the following order.
Similar , observations can be made for aqueous solutions of strong solution bases such as NaOH ,KOH,. Both are completely ionized in dilute aqueous solutions.
* The strongest base that can exist in aqueous solution in OH- ion.
NaOH(s)…….> (NA) + (OH)
*Base stronger that OH- react with H2O to produce OH- and their conjugate acids.
When metal amides such as solution amide, NaNH2 are placed in water, the amide ion, NH2 reacts with water completely.
NH2- + H2O ……….> NH3 + OH